Turning constructor arguments into instance variables [duplicate] - ruby

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Idiomatic object creation in ruby
There are many occaisions when I have an initialize method that looks like this:
class Foo
def initialize bar, buz, ...
#bar, #buz, ... = bar, buz, ...
end
end
Is there a way to do this with a simple command like:
class Foo
attr_constructor :bar, :buz, ...
end
where the symbols represent the name of the instance variables (with the spirit/flavor of attr_accessor, attr_reader, attr_writer)?
I was wondering if there is a built in way or a more elegant way of doing something like this:
class Class
def attr_constructor *vars
define_method("initialize") do |*vals|
vars.zip(vals){|var, val| instance_variable_set("##{var}", val)}
end
end
end
so that I can use it like this:
class Foo
attr_constructor :foo, :bar, :buz
end
p Foo.new('a', 'b', 'c') # => #<Foo:0x93f3e4c #foo="a", #bar="b", #buz="c">
p Foo.new('a', 'b', 'c', 'd') # => #<Foo:0x93f3e4d #foo="a", #bar="b", #buz="c">
p Foo.new('a', 'b') # => #<Foo:0x93f3e4e #foo="a", #bar="b", #buz=nil>

I'd use OpenStruct:
require 'ostruct'
class Foo < OpenStruct
end
f = Foo.new(:bar => "baz")
f.bar
#=> "baz"
Edit: Ah OK, sorry misunderstood you. How about just:
class Foo
def initialize(*args)
#baz, #buz = args
end
end

Would this work for you?
class Foo
def initialize(hash)
hash.each { |k,v| instance_variable_set("##{k}", v) }
end
end

Interesting question. A little meta-programming should take care of it.
module Attrs
def self.included(base)
base.extend ClassMethods
base.class_eval do
class << self
attr_accessor :attrs
end
end
end
module ClassMethods
# Define the attributes that each instance of the class should have
def has_attrs(*attrs)
self.attrs = attrs
attr_accessor *attrs
end
end
def initialize(*args)
raise ArgumentError, "You passed too many arguments!" if args.size > self.class.attrs.size
# Loop through each arg, assigning it to the appropriate attribute (based on the order)
args.each_with_index do |val, i|
attr = self.class.attrs[i]
instance_variable_set "##{attr}", val
end
end
end
class Foo
include Attrs
has_attrs :bar, :buz
end
f = Foo.new('One', 'Two')
puts f.bar
puts f.buz
Of course the downside to this is inflexibility - you have to pass your constructor arguments in a specific order. Of course that's how most programming languages are. Rails people might argue you should instead do
f = Foo.new(:bar => 'One', :baz => 'Two')
which would allow you to pass in attrs in any order, as well as strip away most of the meta-programming. But that is a lot more to type.

Related

Ruby Syntax to Populate a new Object's Instance Variables [duplicate]

I find myself using hash arguments to constructors quite a bit, especially when writing DSLs for configuration or other bits of API that the end user will be exposed to. What I end up doing is something like the following:
class Example
PROPERTIES = [:name, :age]
PROPERTIES.each { |p| attr_reader p }
def initialize(args)
PROPERTIES.each do |p|
self.instance_variable_set "##{p}", args[p] if not args[p].nil?
end
end
end
Is there no more idiomatic way to achieve this? The throw-away constant and the symbol to string conversion seem particularly egregious.
You don't need the constant, but I don't think you can eliminate symbol-to-string:
class Example
attr_reader :name, :age
def initialize args
args.each do |k,v|
instance_variable_set("##{k}", v) unless v.nil?
end
end
end
#=> nil
e1 = Example.new :name => 'foo', :age => 33
#=> #<Example:0x3f9a1c #name="foo", #age=33>
e2 = Example.new :name => 'bar'
#=> #<Example:0x3eb15c #name="bar">
e1.name
#=> "foo"
e1.age
#=> 33
e2.name
#=> "bar"
e2.age
#=> nil
BTW, you might take a look (if you haven't already) at the Struct class generator class, it's somewhat similar to what you are doing, but no hash-type initialization (but I guess it wouldn't be hard to make adequate generator class).
HasProperties
Trying to implement hurikhan's idea, this is what I came to:
module HasProperties
attr_accessor :props
def has_properties *args
#props = args
instance_eval { attr_reader *args }
end
def self.included base
base.extend self
end
def initialize(args)
args.each {|k,v|
instance_variable_set "##{k}", v if self.class.props.member?(k)
} if args.is_a? Hash
end
end
class Example
include HasProperties
has_properties :foo, :bar
# you'll have to call super if you want custom constructor
def initialize args
super
puts 'init example'
end
end
e = Example.new :foo => 'asd', :bar => 23
p e.foo
#=> "asd"
p e.bar
#=> 23
As I'm not that proficient with metaprogramming, I made the answer community wiki so anyone's free to change the implementation.
Struct.hash_initialized
Expanding on Marc-Andre's answer, here is a generic, Struct based method to create hash-initialized classes:
class Struct
def self.hash_initialized *params
klass = Class.new(self.new(*params))
klass.class_eval do
define_method(:initialize) do |h|
super(*h.values_at(*params))
end
end
klass
end
end
# create class and give it a list of properties
MyClass = Struct.hash_initialized :name, :age
# initialize an instance with a hash
m = MyClass.new :name => 'asd', :age => 32
p m
#=>#<struct MyClass name="asd", age=32>
The Struct clas can help you build such a class. The initializer takes the arguments one by one instead of as a hash, but it's easy to convert that:
class Example < Struct.new(:name, :age)
def initialize(h)
super(*h.values_at(:name, :age))
end
end
If you want to remain more generic, you can call values_at(*self.class.members) instead.
There are some useful things in Ruby for doing this kind of thing.
The OpenStruct class will make the values of a has passed to its initialize
method available as attributes on the class.
require 'ostruct'
class InheritanceExample < OpenStruct
end
example1 = InheritanceExample.new(:some => 'thing', :foo => 'bar')
puts example1.some # => thing
puts example1.foo # => bar
The docs are here:
http://www.ruby-doc.org/stdlib-1.9.3/libdoc/ostruct/rdoc/OpenStruct.html
What if you don't want to inherit from OpenStruct (or can't, because you're
already inheriting from something else)? You could delegate all method
calls to an OpenStruct instance with Forwardable.
require 'forwardable'
require 'ostruct'
class DelegationExample
extend Forwardable
def initialize(options = {})
#options = OpenStruct.new(options)
self.class.instance_eval do
def_delegators :#options, *options.keys
end
end
end
example2 = DelegationExample.new(:some => 'thing', :foo => 'bar')
puts example2.some # => thing
puts example2.foo # => bar
Docs for Forwardable are here:
http://www.ruby-doc.org/stdlib-1.9.3/libdoc/forwardable/rdoc/Forwardable.html
Given your hashes would include ActiveSupport::CoreExtensions::Hash::Slice, there is a very nice solution:
class Example
PROPERTIES = [:name, :age]
attr_reader *PROPERTIES #<-- use the star expansion operator here
def initialize(args)
args.slice(PROPERTIES).each {|k,v| #<-- slice comes from ActiveSupport
instance_variable_set "##{k}", v
} if args.is_a? Hash
end
end
I would abstract this to a generic module which you could include and which defines a "has_properties" method to set the properties and do the proper initialization (this is untested, take it as pseudo code):
module HasProperties
def self.has_properties *args
class_eval { attr_reader *args }
end
def self.included base
base.extend InstanceMethods
end
module InstanceMethods
def initialize(args)
args.slice(PROPERTIES).each {|k,v|
instance_variable_set "##{k}", v
} if args.is_a? Hash
end
end
end
My solution is similar to Marc-André Lafortune. The difference is that each value is deleted from the input hash as it is used to assign a member variable. Then the Struct-derived class can perform further processing on whatever may be left in the Hash. For instance, the JobRequest below retains any "extra" arguments from the Hash in an options field.
module Message
def init_from_params(params)
members.each {|m| self[m] ||= params.delete(m)}
end
end
class JobRequest < Struct.new(:url, :file, :id, :command, :created_at, :options)
include Message
# Initialize from a Hash of symbols to values.
def initialize(params)
init_from_params(params)
self.created_at ||= Time.now
self.options = params
end
end
Please take a look at my gem, Valuable:
class PhoneNumber < Valuable
has_value :description
has_value :number
end
class Person < Valuable
has_value :name
has_value :favorite_color, :default => 'red'
has_value :age, :klass => :integer
has_collection :phone_numbers, :klass => PhoneNumber
end
jackson = Person.new(name: 'Michael Jackson', age: '50', phone_numbers: [{description: 'home', number: '800-867-5309'}, {description: 'cell', number: '123-456-7890'})
> jackson.name
=> "Michael Jackson"
> jackson.age
=> 50
> jackson.favorite_color
=> "red"
>> jackson.phone_numbers.first
=> #<PhoneNumber:0x1d5a0 #attributes={:description=>"home", :number=>"800-867-5309"}>
I use it for everything from search classes (EmployeeSearch, TimeEntrySearch) to reporting ( EmployeesWhoDidNotClockOutReport, ExecutiveSummaryReport) to presenters to API endpoints. If you add some ActiveModel bits you can easily hook these classes up to forms for gathering criteria. I hope you find it useful.

How to initialize class in block in Ruby?

I can't figure out the proper block initialize
class Foo
attr_accessor :bar
end
obj = Foo.new do |a|
a.bar = "baz"
end
puts obj.bar
Expect "baz"
instead get nil
What is the proper incantation for block class initializers in ruby?
Another way to make a block initializer would be writing it yourself one:
class Foo
attr_accessor :bar
def initialize
yield self if block_given?
end
end
And later use it:
foo = Foo.new do |f|
f.bar = true
end
My two cents.
Try again:
class Foo
attr_accessor :bar
end
obj = Foo.new.tap do |a|
a.bar = "baz"
end
puts obj.bar
I don't think new can take a block. Never saw it anywhere anyway. Why do you want to initialize in a block ? You can always do obj = foo.new.tap do |a| ... If you really want a block
actually you have a constructor for these purposes:
class Foo
attr_accessor :bar
def initialize(bar = "baz")
#bar = bar
end
end

Customising attr_reader to do lazy instantiation of attributes

(Big edit, I got part of the way there…)
I've been hacking away and I've come up with this as a way to specify things that need to be done before attributes are read:
class Class
def attr_reader(*params)
if block_given?
params.each do |sym|
define_method(sym) do
yield
self.instance_variable_get("##{sym}")
end
end
else
params.each do |sym|
attr sym
end
end
end
end
class Test
attr_reader :normal
attr_reader(:jp,:nope) { changethings if #nope.nil? }
def initialize
#normal = "Normal"
#jp = "JP"
#done = false
end
def changethings
p "doing"
#jp = "Haha!"
#nope = "poop"
end
end
j = Test.new
p j.normal
p j.jp
But changethings isn't being recognised as a method — anyone got any ideas?
You need to evaluate the block in the context of the instance. yield by default will evaluate it in its native context.
class Class
def attr_reader(*params, &blk)
if block_given?
params.each do |sym|
define_method(sym) do
self.instance_eval(&blk)
self.instance_variable_get("##{sym}")
end
end
else
params.each do |sym|
attr sym
end
end
end
end
Here's another alternative approach you can look at. It's not as elegant as what you're trying to do using define_method but it's maybe worth looking at.
Add a new method lazy_attr_reader to Class
class Class
def lazy_attr_reader(*vars)
options = vars.last.is_a?(::Hash) ? vars.pop : {}
# get the name of the method that will populate the attribute from options
# default to 'get_things'
init_method = options[:via] || 'get_things'
vars.each do |var|
class_eval("def #{var}; #{init_method} if !defined? ##{var}; ##{var}; end")
end
end
end
Then use it like this:
class Test
lazy_attr_reader :name, :via => "name_loader"
def name_loader
#name = "Bob"
end
end
In action:
irb(main):145:0> t = Test.new
=> #<Test:0x2d6291c>
irb(main):146:0> t.name
=> "Bob"
IMHO changing the context of the block is pretty counter-intuitive, from a perspective of someone who would use such attr_reader on steroids.
Perhaps you should consider plain ol' "specify method name using optional arguments" approach:
def lazy_attr_reader(*args, params)
args.each do |e|
define_method(e) do
send(params[:init]) if params[:init] && !instance_variable_get("##{e}")
instance_variable_get("##{e}")
end
end
end
class Foo
lazy_attr_reader :foo, :bar, :init => :load
def load
#foo = 'foo'
#bar = 'bar'
end
end
f = Foo.new
puts f.bar
#=> bar

Best Way to Abstract Initializing Attributes

What's the best way to abstract this pattern:
class MyClass
attr_accessor :foo, :bar
def initialize(foo, bar)
#foo, #bar = foo, bar
end
end
A good solution should take superclasses into consideration and be able to handle still being able to have an initializer to do more things. Extra points for not sacrificing performance in your solution.
A solution to that problem already (partially) exists, but if you want a more declarative approach in your classes then the following should work.
class Class
def initialize_with(*attrs, &block)
attrs.each do |attr|
attr_accessor attr
end
(class << self; self; end).send :define_method, :new do |*args|
obj = allocate
init_args, surplus_args = args[0...attrs.size], args[attrs.size..-1]
attrs.zip(init_args) do |attr, arg|
obj.instance_variable_set "##{attr}", arg
end
obj.send :initialize, *surplus_args
obj
end
end
end
You can now do:
class MyClass < ParentClass
initialize_with :foo, :bar
def initialize(baz)
#initialized = true
super(baz) # pass any arguments to initializer of superclass
end
end
my_obj = MyClass.new "foo", "bar", "baz"
my_obj.foo #=> "foo"
my_obj.bar #=> "bar"
my_obj.instance_variable_get(:#initialized) #=> true
Some characteristics of this solution:
Specify constructor attributes with initialize_with
Optionally use initialize to do custom initialization
Possible to call super in initialize
Arguments to initialize are the arguments that were not consumed by attributes specified with initialize_with
Easily extracted into a Module
Constructor attributes specified with initialize_with are inherited, but defining a new set on a child class will remove the parent attributes
Dynamic solution probably has performance hit
If you want to create a solution with absolute minimal performance overhead, it would be not that difficult to refactor most of the functionality into a string which can be evaled when the initializer is defined. I have not benchmarked what the difference would be.
Note: I found that hacking new works better than hacking initialize. If you define initialize with metaprogramming, you'd probably get a scenario where you pass a block to initialize_with as a substitute initializer, and it's not possible to use super in a block.
This is the first solution that comes to my mind. There's one big downside in my module: you must define the class initialize method before including the module or it won't work.
There's probably a better solution for that problem, but this is what I wrote in less than a couple of minutes.
Also, I didn't keep performances too much into consideration. You probably can find a much better solution than me, especially talking about performances. ;)
#!/usr/bin/env ruby -wKU
require 'rubygems'
require 'activesupport'
module Initializable
def self.included(base)
base.class_eval do
extend ClassMethods
include InstanceMethods
alias_method_chain :initialize, :attributes
class_inheritable_array :attr_initializable
end
end
module ClassMethods
def attr_initialized(*attrs)
attrs.flatten.each do |attr|
attr_accessor attr
end
self.attr_initializable = attrs.flatten
end
end
module InstanceMethods
def initialize_with_attributes(*args)
values = args.dup
self.attr_initializable.each do |attr|
self.send(:"#{attr}=", values.shift)
end
initialize_without_attributes(values)
end
end
end
class MyClass1
attr_accessor :foo, :bar
def initialize(foo, bar)
#foo, #bar = foo, bar
end
end
class MyClass2
def initialize(*args)
end
include Initializable
attr_initialized :foo, :bar
end
if $0 == __FILE__
require 'test/unit'
class InitializableTest < Test::Unit::TestCase
def test_equality
assert_equal MyClass1.new("foo1", "bar1").foo, MyClass2.new("foo1", "bar1").foo
assert_equal MyClass1.new("foo1", "bar1").bar, MyClass2.new("foo1", "bar1").bar
end
end
end
class MyClass < Struct.new(:foo, :bar)
end
I know this is an old question with perfectly acceptable answers but I wanted to post my solution as it takes advantage of Module#prepend (new in Ruby 2.2) and the fact that modules are also classes for very simple solution. First the module to make the magic:
class InitializeWith < Module
def initialize *attrs
super() do
define_method :initialize do |*args|
attrs.each { |attr| instance_variable_set "##{attr}", args.shift }
super *args
end
end
end
end
Now let's use our fancy module:
class MyClass
prepend InitializeWith.new :foo, :bar
end
Note that I left our the attr_accessible stuff as I consider that a separate concern although it would be trivial to support. Now I can create an instance with:
MyClass.new 'baz', 'boo'
I can still define an initialize for custom initialization. If my custom initialize take an argument those will be any extra arguments provided to the new instance. So:
class MyClass
prepend InitializeWith.new :foo, :bar
def initialize extra
puts extra
end
end
MyClass.new 'baz', 'boo', 'dog'
In the above example #foo='baz', #bar='boo' and it will print dog.
What I also like about this solution is that it doesn't pollute the global namespace with a DSL. Objects that want this functionality can prepend. Everybody else is untouched.
This module allows an attrs hash as an option to new(). You can include the module in a class with inheritance, and the constructor still works.
I like this better than a list of attr values as parameters, because, particularly with inherited attrs, I wouldn't like trying to remember which param was which.
module Attrize
def initialize(*args)
arg = args.select{|a| a.is_a?(Hash) && a[:attrs]}
if arg
arg[0][:attrs].each do |key, value|
self.class.class_eval{attr_accessor(key)} unless respond_to?(key)
send(key.to_s + '=', value)
end
args.delete(arg[0])
end
(args == []) ? super : super(*args)
end
end
class Hue
def initialize(transparent)
puts "I'm transparent" if transparent
end
end
class Color < Hue
include Attrize
def initialize(color, *args)
p color
super(*args)
p "My style is " + #style if #style
end
end
And you can do this:
irb(main):001:0> require 'attrize'
=> true
irb(main):002:0> c = Color.new("blue", false)
"blue"
=> #<Color:0x201df4>
irb(main):003:0> c = Color.new("blue", true, :attrs => {:style => 'electric'})
"blue"
I'm transparent
"My style is electric"

Idiomatic object creation in ruby

In ruby, I often find myself writing the following:
class Foo
def initialize(bar, baz)
#bar = bar
#baz = baz
end
<< more stuff >>
end
or even
class Foo
attr_accessor :bar, :baz
def initialize(bar, baz)
#bar = bar
#baz = baz
end
<< more stuff >>
end
I'm always keen to minimise boilerplate as much as possible - so is there a more idiomatic way of creating objects in ruby?
One option is that you can inherit your class definition from Struct:
class Foo < Struct.new(:bar, :baz)
# << more stuff >>
end
f = Foo.new("bar value","baz value")
f.bar #=> "bar value"
f.baz #=> "baz value"
I asked a duplicate question, and suggested my own answer there, expecting for a better one, but a satisfactory one did not appear. I will post my own one.
Define a class method like the following along the spirit of attr_accessor, attr_reader, attr_writer methods.
class Class
def attr_constructor *vars
define_method("initialize") do |*vals|
vars.zip(vals){|var, val| instance_variable_set("##{var}", val)}
end
end
end
Then, you can use it like this:
class Foo
attr_constructor :foo, :bar, :buz
end
p Foo.new('a', 'b', 'c') # => #<Foo:0x93f3e4c #foo="a", #bar="b", #buz="c">
p Foo.new('a', 'b', 'c', 'd') # => #<Foo:0x93f3e4d #foo="a", #bar="b", #buz="c">
p Foo.new('a', 'b') # => #<Foo:0x93f3e4e #foo="a", #bar="b", #buz=nil>
Struct
Struct object's are classes which do almost what you want. The only difference is, the initialize method has nil as default value for all it's arguments. You use it like this
A= Struct.new(:a, :b, :c)
or
class A < Struc.new(:a, :b, :c)
end
Struct has one big drawback. You can not inherit from another class.
Write your own attribute specifier
You could write your own method to specify attributes
def attributes(*attr)
self.class_eval do
attr.each { |a| attr_accessor a }
class_variable_set(:##attributes, attr)
def self.get_attributes
class_variable_get(:##attributes)
end
def initialize(*vars)
attr= self.class.get_attributes
raise ArgumentError unless vars.size == attr.size
attr.each_with_index { |a, ind| send(:"#{a}=", vars[ind]) }
super()
end
end
end
class A
end
class B < A
attributes :a, :b, :c
end
Now your class can inherit from other classes. The only drawback here is, you can not get the number of arguments for initialize. This is the same for Struct.
B.method(:initialize).arity # => -1
You could use Virtus, I don't think it's the idiomatic way to do so but it does all the boiler plate for you.
require 'Virtus'
class Foo
include 'Virtus'
attribute :bar, Object
attribute :baz, Object
end
Then you can do things like
foo = Foo.new(:bar => "bar")
foo.bar # => bar
If you don't like to pass an hash to the initializer then add :
def initialize(bar, baz)
super(:bar => bar, :baz => baz)
end
If you don't think it's DRY enough, you can also do
def initialize(*args)
super(self.class.attributes.map(&:name).zip(args)])
end
I sometimes do
#bar, #baz = bar, baz
Still boilerplate, but it only takes up one line.
I guess you could also do
["bar", "baz"].each do |variable_name|
instance_variable_set(:"##{variable_name}", eval(variable_name))
end
(I'm sure there's a less dangerous way to do that, mind you)
https://bugs.ruby-lang.org/issues/5825 is a proposal to make the boilerplate less verbose.
You could use an object as param.
class Foo
attr_accessor :param
def initialize(p)
#param = p
end
end
f = Foo.new
f.param.bar = 1
f.param.bax = 2
This does not save much lines in this case but it will if your class has to handle a large number of param. You could also implement a set_param and get_param method if you want to keep your #param var private.

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